Light fixtures creating various effects are getting more and more used in the entertainment industry in order to create various light effects and mood lighting in connection with live shows, TV shows, sport events or as a part of an architectural installation.
Typically, such variable color light sources comprise a plurality of individually controllable light sources such that each individually controllable light source emits light of a predetermined color. For example, in an RGB system, the variable-color light source may comprise individually controllable light sources of the most common primary colors red, blue, and green. By controlling the relative brightness of the respective individually controllable light sources of the different primary colors almost any color in the visible spectrum may be generated by means of an additive mixing of the respective primary colors, resulting in output light of the desired color and intensity.
U.S. Pat. No. 6,016,038 and U.S. Pat. No. 6,806,659 disclose systems and methods related to LED systems capable of generating light, such as for illumination purposes. The light-emitting LEDs may be controlled by a processor to alter the brightness and/or color of the generated light, e.g., by using pulse-width modulated signals. The disclosed illumination device comprises LEDs including at least two different colors; a switching device, interposed between the LEDs and a common potential reference, including at least two switches corresponding to current paths of the two different color LEDs; a controller that opens and closes the switches according to a predetermined duty cycle. The LEDs of different colors are provided in LED sets each preferably containing serial/parallel array of LEDs of the same color and these LEDs are individual controllable by the controller.
Multi colored illumination devices as disclosed by U.S. Pat. No. 6,016,038 and U.S. Pat. No. 6,806,659 can generate many different colors and the illumination devices are typically instructed to create a target color and/or brightness (e.g. through an input signal indicative of a color and/or brightness). When light from several of such illumination devices are combined into one illumination (e.g. in order to illuminate architectural structure or a large stage area with the same color) color differences might occur even through the different illumination device are instructed to create the same target color. The reason for this is fact that it is difficult to manufacture light sources emitting the exact same color and brightness. This problem is a widely known issue in connection with LEDs and the LED manufacturers have assisted the illumination device providers by pre-sorting the LEDs into smaller ranges of variability prior to shipment. The sorting of the LEDs reduces the color and/or brightness variety of each batch of LEDs and illumination devices manufactured using the same batch of LEDs experience thus less color and/or brightness variations. However acceptable color and brightness rendering is still a demanding task because even the presorted batches of LED have a sizeable range of the performance variations and the cost of pre-sorted batches are much higher than regular batches of LED. Further the end user combining multiple number of illumination devices may have illuminations from different production batches where different batches of LED have been used on the color and/or brightness variation of such illumination devices are even bigger.
It is known that it is possible to compensate for the differences in color and/or brightness of the type/color of LED in a multicolor illumination device by calibrating the illumination in connection with the manufacturing process. The calibration data define color and/or brightness properties of the LEDs and the illumination device is adapted to adjust the color and/or brightness of the LEDs based on the calibration data. Differences in color and/or brightness of the LEDs can hereby be taking into account when driving the illumination device. For instance U.S. Pat. No. 8,013,281 and WO 2007/062662 describes such systems.
U.S. Pat. No. 8,013,281 disclose a system and method for calibrating light output from a LED. The system includes a support on which a LED is positioned, a photosensor to measure the light output from the LED, and means for calibrating and adjusting the light output of the LED. Calibration is accomplished by measuring the light output from the LED, comparing such output against a reference value, and adjusting the measured output against the reference value.
WO 2007/062662 discloses a control device for controlling a variable-color light source, the variable-color light source comprising a plurality of individually controllable color light sources. The control device comprises a control unit for generating, responsive to an input signal indicative of a color and a brightness, respective activation signals for each of the individually controllable color light sources. The control unit is configured to generate the activation signals from the input signal and from predetermined calibration data indicative of at least one set of color values for each of the individually controllable light sources.
Further it is known that the color and/or brightness of the LEDs changes with the junction temperature of LED. Typically the LED manufactures provides information of how the color and/or brightness of the LED changes according the junction temperature. As a consequence the illumination is further adapted to adjust the color and/or brightness of the LEDs based on information provided by the manufactures and the junction temperature. However it is difficult to obtain a precise junction temperature of the LED, as it typical are estimated from a temperature measurement of the PCB whereon the LED is mounted and a temperature formula provided by the LED manufacture. There may thus still exits color and/or brightness variations in such illumination devices.
U.S. Pat. No. 7,626,345 discloses a manufacturing process for storing measured light output internal to an individual LED assembly, and an LED assembly realized by the process. The process utilizes a manufacturing test system to hold an LED light assembly a controlled distance and angle from the spectral output measurement tool. Spectral coordinates, forward voltage, and environmental measurements for the as manufactured assembly are measured for each base color LED. The measurements are recorded to a storage device internal to the LED assembly. Those stored measurements can then be utilized in usage of the LED assembly to provide accurate and precise control of the light output by the LED assembly.
Illumination devices where the color and/or brightness of the LEDs are regulated based on a live/online measurement of the outgoing light are also known. U.S. Pat. No. 6,894,442 disclose a light source and method for controlling the same. The light source utilizes a light generator that generates a light signal of a wavelength at an intensity that is set by a control signal. The control signal is controlled by a servo that monitors the light output of the light generator and compares the monitored value with a target value. When the target value is changed, the control signal is initially replaced by a predicted control signal based on the new target value rather than the error signal generated in the servo. This provides time for the servo to adjust to the new target value. In one embodiment, the control signal includes a periodic signal that switches between a value that causes the light generator to generate light of the wavelength and a second value at which the light generator does not generate light of the wavelength.
Systems, like the one disclosed by U.S. Pat. No. 6,894,442, where the intensity of the light sources are regulated based live/online measurement of the light generated by the light sources are also disclosed by WO02/080625, US2007/0108846, WO2008/153642 and WO 02/47438 (all briefly described below). In general, such system are complicated to implement as it requires that the photo sensors do not measure ambient light or that the system account for ambient light. However, the is very difficult in connection with entertainment shows where the ambient light changes often as light from neighboring lamps may hit the light fixture and thus influence the light measurements. The photo sensors themselves may also introduce error if they also are not calibrated correctly and/or or provides measurements with large tolerances. The photo sensors and required techniques also adds thus extra costs to the light fixture, for instance as accurate photo sensor with low tolerances are expensive.
WO 02/080625 discloses a system for controlling a RGB based LED luminary which tracks the tristimulus values of both feedback and reference whereby the forward currents driving the LED luminary are adjusted in accordance with the errors between the feed tristimulus values and the reference tristimulus values until the errors are zero.
US 2007/0108846 disclose a method and system for controlling the chromaticity and luminous flux output of a digitally controlled luminaire. The luminaire comprises one or more light-emitting elements and one or more light sensors which can provide optical feedback, wherein this optical feedback is filtered to remove undesired frequencies. The method and system comprises a control system which can sample the filtered signals from the light sensors according to a predetermined feedback sampling frequency scheme, wherein this scheme is specifically configured to provide sufficient iterations of the feedback loop to be performed for adjustment of the chromaticity and luminous flux output of the light-emitting elements, without perceptible visual flicker or momentary chromaticity shifts.
WO 2008/153642 discloses a method of calibrating a lighting panel including a plurality of segments, a respective segment configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles, includes activating the plurality of segments to simultaneously emit the first and second colors of light. A combined light output for the plurality of segments is measured at a measurement location to obtain aggregate emission data. Separate emission data for the first and second colors of light is determined based on the aggregate emission data. For example, the separate emission data for the first and second colors of light may be derived based on extrapolation of the aggregate emission data and expected emission data for the first and second colors of light. Related calibration systems are also discussed.
WO 02/47438 discloses, a LED luminary system for providing power to LED light sources to generate a desired light color comprises a power supply stage configured to provide a DC current signal. A light mixing circuit is coupled to said power supply stage and includes a plurality of LED light sources with red, green and blue colors to produce various desired lights with desired color temperatures. A controller system is coupled to the power supply stage and is configured to provide control signals to the power supply stage so as to maintain the DC current signal at a desired level for maintaining the desired light output. The controller system is further configured to estimate lumen output fractions associated with the LED light sources based on junction temperature of the LED light sources and chromaticity coordinates of the desired light to be generated at the light mixing circuit. The light mixing circuit further comprises a temperature sensor for measuring the temperature associated with the LED light sources and a light detector for measuring lumen output level of light generated by the LED light sources. Based on the temperatures measured, the controller system determines the amount of output lumen that each of the LED light sources need to generate in order to achieve the desired mixed light output, and the light detector in conjunction with a feedback loop maintains the required lumen output for each of the LED light sources.